A nuclear reactor core, and in particular a boiling water reactor, includes a plurality of individual fuel assemblies that have different characteristics that affect the strategy for operation of the core. The use of coolant in the core also affects the strategy for operation of the core. Coolant is introduced in the core to cool the core, to be transitioned into steam as a working fluid for energy generation, and to provide thermal neutron source aid in the nuclear reaction.
Fuel support members support the lower end of the fuel assemblies and provide a flow path for coolant to enter the lower end of the fuel assemblies. For most fuel assemblies, the flow path proceeds vertically and upward as it approaches a fuel support member. The coolant then flows horizontally into a side entry inlet orifice for entry into the fuel support member, then vertically and upward again for the approach to the fuel assembly. The flow path in this final approach to the most of the fuel assemblies is not symmetric with respect to the fuel assembly flow path. In fact, only some peripheral fuel assemblies experience symmetric flow through the fuel support member and into the fuel assembly. The flow enters the fuel assembly through the lower tie plate, which supports fuel rods and is also shaped to facilitate seating of the fuel assembly into the fuel support member.
One of the key design considerations for a fuel assembly is the minimum critical power ratio (MCPR), which is a limit selected to protect the fuel assembly from undergoing a boiling transition, which would expose the fuel assembly to excessive temperature. The MCPR is directly related to the flow quality, which is a positive linear function of the fuel assembly power divided by fuel assembly flow rate. High power fuel assemblies require more flow to maintain the same MCPR but, with current side entry inlet orifice layouts, generally receive less flow than the average fuel assembly. This is because the interior fuel support members have the same inlet orifice size, and thus the same loss coefficient. High power fuel assemblies produce more steam, which increases the fuel assembly pressure losses and reduces the flow relative to the average fuel assembly. Consequently, high power fuel assemblies produce especially high quality flow, while low power fuel assemblies produce especially low quality flow.
The fuel support members include an inlet orifice to control coolant distribution between the fuel assemblies and to assist in thermal-hydraulic stability performance of the reactor core. Generally, the inlet orifices of fuel support members were designed at the time of reactor construction to have loss coefficients optimized for then-existing modes of core operation and fuel designs. There have since been changes in fuel designs and core operation, and the orifices are no longer optimized for current fuel assembly designs and/or core operation. Generally, there are two orifice loss coefficients: a high loss coefficient for the fuel assemblies around the periphery of the core and a lower one for all other fuel assemblies. As the fuel assemblies on the periphery of the core have significant neutron leakage, the power in these fuel assemblies is relatively low. The flow to these assemblies is reduced due to orificing, but not sufficiently reduced for current fuel assembly designs and core operation.
The non-uniform exit quality distribution combined with lower values of average exit quality due to non-optimum orificing reduces the capability for reactor systems such as steam separators and dryers to operate effectively. This results in higher amounts of moisture carryover to the turbine, which causes increased erosion damage to turbine blades and steam cycle piping. Erosion damage is costly as it impacts operational life of the affected components and reduces efficiency of the plant's thermodynamic cycle. The moisture also carries small, irradiated particles that collect in various locations of the turbine system. These particles become the sources of higher radiation exposure in the balance of the plant.
Additionally, non-optimum orificing incurs a higher fuel cycle cost. As noted above, high power fuel assemblies are significantly more limiting than other fuel assemblies in terms of MCPR. This limitation translates into non-optimal fuel assembly design and reactor operating conditions in order to comply with the MCPR limits.
Previous attempts to address these problems involve changing the design of the fuel support member. However, the fuel support members are typically installed when the reactors are built and are not easily replaced. Moreover, design changes such as replacing members are relatively permanent, and provide a static adjustment to loss coefficient. As such, permanent modifications do not facilitate customized and adjustable core control strategy planning and implementation. Accordingly, there is a need for a dynamic solution to the problem of control of coolant flow through fuel assemblies.